What is the Mempool?
Short for "memory pool." The waiting area where unconfirmed transactions sit before being included in a block. When the mempool is full, transactions with higher fees get priority.
Why It Matters
The mempool is where market forces determine transaction fees. When Bitcoin usage is light, the mempool is nearly empty and you can get your transaction confirmed quickly with a minimal fee. When Bitcoin usage is heavy and block space becomes scarce, the mempool fills up with transactions competing for limited block space. Miners prioritize transactions with higher fees, so if you want faster confirmation during high congestion, you need to pay more. Understanding the mempool helps you understand transaction fees. Paying the minimum fee when the mempool is full will result in your transaction sitting unconfirmed for hours or days. The mempool is also evidence of Bitcoin's adoption—a full mempool indicates high demand to transact.
How It Works
When you send a Bitcoin transaction, it gets broadcast to the network and enters nodes' mempools. Miners collect transactions from the mempool, prioritizing by fee (usually measured in satoshis per byte of transaction size). When a miner constructs a new block, they fill it with the highest-fee transactions from their mempool, up to the block size limit. Once a transaction is included in a block, it leaves the mempool and becomes confirmed. If you want to replace an unconfirmed transaction with a higher fee, you can use Replace-by-Fee (RBF) to broadcast a new version of the same transaction with a higher fee, which will typically get priority.